1. Field of the Invention
The invention relates to a friction bolt.
2. Description of the Related Art
Such friction bolts (trademark designation of the Atlas Copco “Swellex”) are known. For example, reference can be made to U.S. Pat. No. 4,459,067 A, WO 2005/073510 A, WO 2005/119009 A, WO 2006/066288 A and WO 2008/019409 A.
The known friction bolts are roof bolts (rock bolts), which have a tube that is folded inward in the longitudinal direction, which tube is introduced into a borehole and is widened by increasing the pressure inside the folded tube, so that the outside surface of the tube is applied to the inside surface of the borehole and the bolt is thus fixed (positively) in the borehole.
The known friction bolts have a cross-sectional shape that is reproduced in FIG. 1. In this known cross-sectional shape, the inward-folded part of the tube has a shape that is essentially concentric to the (circular) shape of the outer part of the tube. In addition, an undercut is produced by the narrow gap at the beginning (root) of the inward-folded part of the tube and the widening of the inward-folded part. This undercut is problematic in many respects.